Hitchcocks Forum

Hi all... as part of a 35000 km gearbox overhaul I'm replacing both the layshaft and the bushes for the layshaft (new ones: our hosts phosphor bronze counterparts). The bush for the kickstart spindle is already replaced, and now the layshaft doesn't fit. Bush inner diameter is 18,15 mm; new layshft kickstart spindle end is 18,25 mm. What should the optimal tolerance be? And how do I fix it? Shaving off some material from the bush or the layshaft? And how to do it? I'm planning to do the case end bush next week. Anything in particular I should be observant about... After Bullet Whisperer mentioned a gearbox lockup at 100 mph because of a too tight bush/layshaft combination I'm pretty determined on getting this right. So, advice is much appriciated... regards Jacob

I would teams the bush with an adjustable reamer untill you achieve an easy sliding fit.

Damn spell checker, ream the bush.

Hi Pete... thanks for reply. The reamer thing was my initial thought, but where to get such a tool. I have never seen them before this small here in Denmark. Well, the internet might come in handy once again for this search. What about tolerance? Is it just a question about feeling an easy sliding fit or does it have a value in mm's?

The bushes are finished machined to be the correct fit once it has been pressed into the Kickstart.


I have never encountered a tight one, more often and more likely to be too loose as a result of wear to the layshaft itself. In which event the new bush does not last long.
I usually replace the lay shaft at the same time and occasionally get the old shaft refurbished. Metal sprayed and reground, but new ones are cheaper...

Hi Tim... as written in my intro to the problem both bush and layshaft are new. The layshaft is a RE factory part and the bush is our hosts part # H15. Before fitting the bush to the kickstart spindle it had a tight fit to the layshaft and now it doesn't fit. The bush for the case end has a nice fit before fitting. I plan to replace the case end this weekend. After this I know if it still has a nice fit. I have tried to upload pictures, but don't know if it works... Jacob

Well, the pix upload was no success, but they are pictures of the new and old layshaft (kickstarter end) and the new bush in the kickstarter spindle. I happen to have another kickstarter spindle with a new oem bush lying around. The new layshaft has a nice fit in this one. I guess some careful reaming can do the job.

Papasmurf, thank you for helping me out on making the pictures visible. I hope the one on the old layshaft shows why I had to do something about new bushes.

The trick for the pictures is to copy and paste the HTML code and NOT the image code, which is what most forums use, but not this one...



I think your 18.25mm is actually 23/32" of an inch, and if the blades of an H7 parallel expanding reamer (21/32"-23/32" range) won't reach far enough into the blind hole you will need a plain reamer with cutting flutes going right to one end. I had a quick internet search last night, and while there seem to be no hand reamers the correct size which you can use with a normal tap wrench, they are available on ebay UK as machine reamers with a morse taper fitting for use with drill presses and lathes, so a machine shop could help you out if you get one - unless you already have a morse taper to tap wrench adapter lying around!



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